error[E0463]: can't find crate for `core`
   |
   = note: the `thumbv7em-none-eabihf` target may not be installed
   = help: consider downloading the target with `rustup target add thumbv7em-none-eabihf`
   = help: consider building the standard library from source with `cargo build -Zbuild-std`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0463`.